Inverted Style Balun with DC Isolated Differential Ports

ABSTRACT

The present invention is directed to a balun that includes a first coupler structure having a first port of a balanced port pair and an unbalanced port. A second coupler structure includes a second port of the balanced port pair. The second coupler port structure being connected to the first coupler structure such that the second port of the balanced port pair is DC isolated from the first port of the balanced port pair without decoupling components.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on U.S. Provisional Patent Application60/764,715 filed on Feb. 2, 2006, the content of which is relied uponand incorporated herein by reference in its entirety, and the benefit ofpriority under 35 U.S.C. § 119(e) is hereby claimed, this applicationalso claims priority to U.S. patent application Ser. No. 11/419,091filed on May 18, 2006, the content of which is relied upon andincorporated herein by reference in its entirety, and the benefit ofpriority under 35 U.S.C. § 120 is hereby claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to radio-frequency (RF) and/ormicrowave components, and particularly to RF and/or microwave coupledtransmission line components.

2. Technical Background

Communication systems typically require a number of sub-systems andcomponents to convert baseband signals into RF signals for subsequenttransmission over a communication channel. Conversely, RF signalsreceived via the communication channel must be converted into basebandsignals for use by the user and/or subscriber. Examples of such systemsare ubiquitous and include cell phones, cable television converters,satellite television converters, etc.

It is often the case wherein one stage of the communication systememploys differential (i.e., balanced signals) signals and a subsequentstage unbalanced signals. A differential signal includes two signalpaths, each being 180° out of phase with the other. An unbalanced lineis simply implemented as a single signal path. For example, certainantennas are balanced structures that require a balanced feed. However,the system may be such that the signal source is an unbalanced RFtransmitter. This situation may also present itself in the oppositedirection as well. A push/pull amplifier, for example, may provide abalanced differential signal for subsequent use by an unbalancedantenna. As those of ordinary skill in the art will appreciate, a balunis typically used to couple a balanced signal source to an unbalancedload (e.g., an antenna) or vice-versa. The word “balun” is shorthand fora balanced-unbalanced network.

Baluns are typically implemented using several coupled transmissionlines, i.e., directional couplers. Couplers are four-port passivedevices that are commonly employed in radio-frequency (RF) and microwavecircuits and systems. A coupler may be implemented by disposing twoconductors in relative proximity to each other such that an RF signalpropagating along a main conductor is coupled to a secondary conductor.The RF signal is directed into an input port connected to the mainconductor and power is transmitted to an output port disposed at thedistal end of the main conductor. An electromagnetic field is coupled tothe secondary conductor and the coupled RF signal is directed into anoutput port disposed at an end of the secondary conductor. The outputsignals are, of course, 90° out of phase with each other. An isolationport is disposed at the other end of the secondary conductor. The termisolation port refers to the fact that, ideally, the RF signal is notavailable at this port. At the isolation port, the incident signal andthe coupled signal are substantially out of phase with each other andcancel each other out.

Those of ordinary skill in the art will appreciate that balunperformance, weight, form factor and volume are important issues formost implementations. One commonly known balun implementation isreferred to as a Marchand balun. The Marchand balun includes a mainhalf-wavelength transmission line coupled to two quarter-wavelengthtransmission lines. The unbalanced port is connected to thehalf-wavelength structure. The quarter-wavelength transmission linesprovide the differential signal ports. Each differential signal portaccommodates a signal that is equal in amplitude and opposite in phaseto the other differential port. The Marchand balun is limited in that itsupports wideband applications only when the unbalanced impedance islower than the impedance of the balanced ports. Typical impedancetransformation ratios are 1:2 or 1:4. A variation of the Marchand balunis known as the Merrill balun.

The Merrill balun may be thought of as an inverted Marchand balunbecause the balanced signals are provided at either end of thehalf-wavelength structure. The unbalanced port is disposed at one end ofone of the quarter wavelength transmission lines. The other quarterwavelength transmission line is grounded at both ends. Thehalf-wavelength structure and the quarter wavelength elements may beimplemented using stripline segments formed by disposing a layer ofconductive material on a dielectric substrate. While the performance ofthe Merrill balun, as measured by insertion loss and return loss over apredetermined bandwidth, is adequate, there are drawbacks associatedwith this balun implementation. The Merrill balun is limited in that itsupports wideband applications only when the balanced impedance is lessthan or equal to the unbalanced impedance. For example, typicalimpedance transformation ratios are 1:1 or 2:1. This is another reasonwhy Merrill baluns are referred to in some quarters as inverted Marchandbaluns. In many designs, the electrical length and the even-modeimpedance are essentially fixed, only the odd-mode impedance may bemanipulated to optimize performance. One drawback of both the Marchandand Merrill baluns relates to the excessive line-widths of the striplinestructures at certain odd-mode impedance values.

In certain applications, system designers are requiring that thebalanced ports of the balun are isolated from each other and ground. Ineach of the examples discussed above, there are direct current (DC)paths between the balanced ports and/or ground. As those of ordinaryskill in the art will understand, DC isolation is typically implementedby coupling the differential ports of the balun to the balanced signalsource/sink via decoupling capacitors. Thus, size reductions may berealized if decoupling capacitors could be eliminated from the design.

What is needed is a balun implementation having an isolated balancedport while conforming to a desired form factor for a desired performancespecification.

SUMMARY OF THE INVENTION

The present invention addresses the needs described above by providingan isolated balanced port while conforming to a desired form factor fora desired performance specification.

One aspect of the present invention is directed to a balun that includesa first coupler structure having a first port of a balanced port pairand an unbalanced port. A second coupler structure includes a secondport of the balanced port pair. The second coupler structure isconnected to the first coupler structure such that the second port ofthe balanced port pair is DC isolated from the first port of thebalanced port pair without decoupling components.

In another aspect, the present invention is directed to a balun thatincludes a first coupler structure having a first port of a balancedport pair and an unbalanced port. A second coupler structure includes asecond port of the balanced port pair. The second coupler structure isconnected to the first coupler structure such that the first port of thebalanced port pair and the second port of the balanced port pair areisolated from ground potential without decoupling components.

In yet another aspect, the present invention is directed to a devicethat includes a first coupler structure having a first portion of afirst balanced port pair, a first portion of a second balanced port pairand an unbalanced port. A resistive element is connected to the firstcoupler structure. A second coupler structure includes a second portionof the first balanced port pair and a second portion of the secondbalanced port pair. The second coupler structure is connected to thefirst coupler structure by way of the resistive element such that thefirst and second portions of the first balanced port pair and the firstand second portions of the second balanced port pair are isolated fromground potential without decoupling components.

In yet another aspect, the present invention is directed to a balunhaving a first coupler structure including a first port of a balancedport pair and an unbalanced port. The first coupler structure includes afirst transmission line layer coupled to a second transmission linelayer and a third transmission line layer coupled to the secondtransmission layer. The second transmission line layer is disposedbetween the first transmission line layer and the third transmissionline layer. A second coupler structure includes a second port of thebalanced port pair. The second coupler structure also includes a fourthtransmission line layer coupled to a fifth transmission line layer and asixth transmission line layer coupled to the fifth transmission layer.The fifth transmission line layer is disposed between the fourthtransmission line layer and the sixth transmission line layer. The firsttransmission line layer is connected to the sixth transmission linelayer and the third transmission line layer is connected to the fourthtransmission line layer such that the first port of the balanced portpair is DC isolated from the second port of the balanced port pair.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of the inventionand are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention and are incorporated in, and constitute a part of, thisspecification. The drawings illustrate various embodiments of theinvention, and together with the description serve to explain theprinciples and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a vertical interdigital coupler inaccordance with one embodiment of the present invention;

FIG. 2 is a plan view of a transmission line layer of a verticalinterdigital coupler in accordance with the present invention;

FIG. 3A-3B are diagrammatic depictions of the even mode and odd modecoupling field lines for the coupler depicted in FIG. 2;

FIG. 4 is a diagram illustrating the coupler cross-sectional area inaccordance with the present invention;

FIGS. 5A-5C are schematic diagrams illustrating vertical interdigitalcoupler design considerations;

FIG. 6 is a balun in accordance with one embodiment of the presentinvention;

FIG. 7 is a chart illustrating the performance of the balun depicted inFIG. 6;

FIG. 8 is a chart illustrating the insertion loss of the balun depictedin FIG. 6 as a function of frequency and even-mode impedance;

FIG. 9 is a chart illustrating the insertion loss of the balun depictedin FIG. 6 as a function of frequency and odd-mode impedance;

FIG. 10 is a balun in accordance with another embodiment of the presentinvention;

FIG. 11 is a chart illustrating the insertion loss of the balun depictedin FIG. 10 as a function of frequency and even-mode impedance;

FIG. 12 is a chart illustrating the insertion loss of the balun depictedin FIG. 10 as a function of frequency and odd-mode impedance;

FIG. 13 is a balun in accordance with yet another embodiment of thepresent invention;

FIG. 14 is a chart illustrating the performance of the balun depicted inFIG. 13;

FIG. 15 is a chart illustrating the insertion loss of the balun depictedin FIG. 13 as a function of frequency and even-mode impedance;

FIG. 16 is a chart illustrating the insertion loss of the balun depictedin FIG. 13 as a function of frequency and odd-mode impedance;

FIGS. 17A-17E are charts illustrating the design tradeoffs of thepresent invention relative to a Merrill balun;

FIG. 18 is a power divider in accordance with another embodiment of thepresent invention;

FIG. 19 is a combiner in accordance with yet another embodiment of thepresent invention;

FIG. 20 is a perspective view of the device depicted in either FIG. 6,10, 13, 18 or 19 in accordance with the present invention; and

FIG. 21 is an exploded view of the device depicted in either FIG. 6, 10,13, 18 or 19 in accordance with the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the present exemplaryembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.One embodiment of the balun of the present invention is shown in FIG. 6,and is designated generally throughout by reference numeral 100.

In accordance with the invention, initially referring to FIG. 6, thepresent invention for a balun 100 includes a coupler structure 10 havingone port (2) of a balanced port pair and an unbalanced port (1). Anothercoupler structure 10′ includes the second port (3) of the balanced portpair. Coupler structure 10′ is connected to coupler structure 10 suchthat the balanced ports, i.e., port 2 and port 3 are DC isolated fromeach other without using any decoupling components. It will be apparentto those of ordinary skill in the pertinent alt that modifications andvariations can be made to the coupler structures (10, 10′) of thepresent invention depending on performance and form factor issues. Forexample, the coupler structures (10, 10′) may be implemented usingvertical interdigital couplers. In another embodiment, the couplerstructures (10, 10′) are implemented using edge couplers.

Referring back to FIG. 1, a schematic diagram of a cross-sectionalportion of a vertical interdigital coupler 10 in accordance with oneembodiment of the present invention is disclosed. While the referencenumeral for the coupler structure employed in the following discussionis ten (10), the discussion equally applies to coupler 10′ in balun 100.Coupler 10 is a vertical interdigital coupler that includes port 1, port2, port 3, and port 4. In this embodiment, the vertical interdigitalcoupler includes three coupled transmission lines, i.e., transmissionline 14 is interposed between two transmission lines 12. Eachtransmission line 12 is disposed on a dielectric substrate 16 andcoupled between port 1 and port 2 to form a transmission line layer.Each of the transmission lines 14 are also disposed on a dielectricsubstrate 16 to form an adjacent transmission line layer. Transmissionlines 14 are coupled between port 3 and port 4.

In general, vertical interdigital couplers may be implemented bydisposing transmission line layers 14 in alternating layers withtransmission line layers 12 to form a total of N transmission linelayers. Transmission lines 12 and transmission lines 14 are disposed ina predetermined vertical position relative to each other. In oneembodiment, transmission lines 12 may be vertically aligned withtransmission lines 14 to effect maximum coupling. In other embodiment,transmission lines 14 are vertically offset from transmission lines 12to obtain a different degree of coupling. In other words, the verticalgeometric configuration may be adjusted to obtain a predeterminedcoupling constant. In accordance with the present invention, N is aninteger value that is greater than or equal to three (3).

In the balun structure of the present invention, N is typically equal tothree. N may be selected for a variety of reasons including couplingvalue, form factor considerations and etc. The alternating layers oftransmission line layers 12 and transmission line layers 14 aretypically disposed between a pair of ground plates 18. In certainembodiment, however, the ground plates 18 are unnecessary. Each secondtransmission line is disposed in a predetermined position relative to acorresponding first transmission line within the structure. However,those of ordinary skill in the alt will understand that the balunstructures of the present invention should not be deemed as beinglimited to coupler structures having only three layers.

Referring to FIG. 2, a plan view of a transmission line layer 12 isshown. FIG. 2 is equally applicable to line 14. As noted above,transmission lines 12, 14 are configured to conform to a predeterminedgeometric configuration. In this case, transmission line 12 is disposedin a folded square geometry. The length of transmission line 12 isapproximately 68 mm. The geometric configuration, therefore, refers tothe shape of the transmission line in plan view, the width of theconductors, the thickness of the conductors, the thickness of thedielectric, and all the various spacing dimensions. It will be apparentto those of ordinary skill in the pertinent art that modifications andvariations can be made to predetermined geometric configuration of thepresent invention depending on the desired coupling and the specifiedvolume/dimensional form factor requirements. In the illustrated example,transmission line 12 is disposed on substrate 16 in a folded squareconfiguration. On the other hand, those of ordinary skill in the artwill understand that the geometric configuration may be any suitableshape, such as linear, rectangular, non-linear, spiral or circular, andetc. The geometric pattern may include meandered line segments and othersuch geometries.

FIG. 3A is a diagrammatic depiction of even mode coupling field linesfor the coupler depicted in FIG. 2. As those of ordinary skill in theart will appreciate, even mode coupling refers to the scenario whereintransmission line 12 and transmission line 14 are at the same electricalpotential. By definition, there is no coupling between transmissionlines 12 and the transmission line 14 sandwiched therebetween. However,an electric field is established between transmission lines 12, 14 andthe ground plates 18.

FIG. 3B is a diagram of the odd mode field lines. In the odd-mode,transmission lines 12 and transmission line 14 are at differentpotentials. Accordingly, an electric field is generated betweentransmission lines 12 and transmission line 14. FIGS. 3A-3B furtherillustrate that the arrangements depicted herein may be approximated asa parallel plate capacitor configuration. Thus, the capacitance isproportional to the area of the transmission line broad side, i.e., thelength and width of the coupled broadside. FIG. 3B is noteworthy becauseit illustrates the improved coupling characteristics of the presentinvention relative to conventional devices. Note that transmission line14 is coupled to transmission lines 12 from both sides of thetransmission line.

Those of ordinary skill in the art will understand that three layerstructures are inherently asymmetrically coupled devices. Therefore, theuse of Even and Odd mode impedances herein is a simplified approximationof the asymmetric nature of the vertical interdigital couplerstructures. However, those of ordinary skill in the art will alsounderstand that using asymmetric mathematical models to explain thepresent invention overly, and significantly, complicates the disclosurewith very limited benefit. In fact, those of ordinary skill in the artwill understand that the compact structural designs disclosed hereinmust be finalized using three-dimensional (3D) electromagneticsimulators whether asymmetrical or symmetrical models are employed.

FIG. 4 is a diagram showing coupler cross-sectional designconsiderations in accordance with the present invention. As notedpreviously, the vertical interdigital broadside coupler 10 may beminiaturized and engineered to be disposed in a physical form factorhaving predetermined dimensional specifications. In the exampleprovided, there are three vertically broadside coupled transmissionlines 12, 14, and 15, i.e., N=3. Dimension d is the vertical distancebetween broadside coupled transmission lines 12, 14, 15. Dimension h isthe vertical distance from each outermost conductor 14 to the closestground plane 18 (if present). Dimension t is the vertical height of eachconductor 12, 14. Dimension s is the horizontal spacing between adjacentsegments in a given transmission line conductor. Dimension w is thewidth of each conductor, i.e., the dimension in the horizontal plane ofFIG. 4. Finally, m is the ratio between conducting and non-conductingmaterial in the horizontal direction, wherein:

$\begin{matrix}{m \approx \frac{w}{w + s}} & (2)\end{matrix}$

The total ground plane spacing of the stripline structure, not includingconductor thickness is:

b _(N)=2h+(N−1)d  (3)

The total ground plane spacing of the stripline structure including theconductor thickness is:

B _(N)=2h+(N−1)d+Ntm  (4)

The cross sectional area occupied by a coupled section is therefore:

A _(N) =B _(N)(s+w)=(s+w)(2h+(N−1)s+Ntm)  (5)

Equation (5) is an approximation that assumes that the structure has anelectrical wall interposed between each vertical conductor group. Thisapproximation is reasonable for tightly spiraled structures with X-Ydimension much smaller than one quarter wavelength (λ/4). Thus, thecapacitances can be approximated to that of parallel plate capacitance:

$\begin{matrix}{C_{P} = {ɛ_{0}ɛ_{r}\frac{lw}{d_{CP}}}} & (6)\end{matrix}$

The dimension l is the length of the transmission lines and d_(CP) isthe distance between the plates.

$\begin{matrix}{{{{If}\mspace{14mu} C_{x}} = {ɛ_{0}ɛ_{r}{lw}}},{\lbrack{Fm}\rbrack = {8.854\; ɛ_{r}{lw}}},\lbrack{pFm}\rbrack,} & (7) \\{{then};\mspace{14mu} {C_{P} = {\frac{C_{x}}{d_{cp}}.}}} & (8)\end{matrix}$

C_(x) is employed in the even and odd mode capacitance equations derivedherein. Those of ordinary skill in the art will understand that theconstants ε₀ and ε_(r) in equation (7) refer to the permittivity of thedielectric material. Permittivity is a measure of a dielectricmaterial's response to an applied electric field. In particular, if thepermittivity of a first dielectric material is larger than thepermittivity of a second dielectric material, the first material willstore a greater charge for a given applied electric field. As equation(7) suggests, permittivity is proportional to capacitance. Thus, thefirst dielectric material will have a greater capacitance. Note alsothat ε₀, the permittivity of free space is 8.8541878176×10⁻¹² farads permeter (F/m). Hence, [pFm] is used to denote “pico Farads per meter” inequation (7).

FIGS. 5A-5C are schematic diagrams illustrating vertical interdigitalcoupler design considerations in accordance with a three-layerembodiment of the present invention. FIG. 5B is a schematic showingequivalent odd-mode capacitances for the three layer coupler design ofthe present invention. FIG. 5C is a schematic showing the equivalenteven-mode capacitances.

$\begin{matrix}{C_{o\; 3} = {{2C_{d}} = \frac{2C_{x}}{d}}} & (13)\end{matrix}$

Note that the odd-mode capacitance does not depend on the strip lineheight. This implies that the stripline ground planes may be removedwithout any adverse consequences (relative to the odd mode). In otherwords, this design is an approximation of a coax cable. Also of note isthat the even-mode capacitance is identical to the conventional 2-layerbroadside coupler. In fact, the even-mode capacitance does not depend onthe value of N.

As embodied herein and depicted in FIG. 6, a balun 100 in accordancewith one embodiment of the present invention is disclosed. Couplerstructure 10 includes coupled transmission line layers 12, 14, and 15.Transmission line layer 14 is disposed between transmission line layer12 and transmission line layer 15. Coupler structure 10′ is very similarto coupler structure 10. It includes coupled transmission line layers12′, 14′ and 15′. Transmission line layer 14′ is sandwiched between thetransmission line layer 12′ and transmission line layer 15′. Balun 100,of course, includes an unbalanced port 1 and a balanced port. Thebalanced port further includes “in-phase” port 2 and “quadrature phase”port 3. The terms in-phase and quadrature, as used herein, merely referto the fact that port 2 and port 3 support signals that are ofsubstantially equal in amplitude and 180° out of phase with each other.On the other hand, the terms “in-phase” and “quadrature”, as usedherein, do not necessarily mean that the in-phase port is substantiallyin phase with the unbalanced port 1. In fact, in various embodiments,the in-phase port may be +90° out of phase with the unbalanced portwhereas the quadrature port may be −180′ out of phase with theunbalanced port. The present invention should not be construed as beinglimited to any of these examples.

In-phase port 2 is connected to transmission line layer 12 andtransmission line layer 15. Quadrature port 3 is connected totransmission line layer 12′ and transmission line layer 15′. Theinternal ends of coupler 10 transmission lines (12, 15) are connected tothe internal ends of coupler 10′ transmission lines (12′, 15′). One endof transmission line 14 is connected to the unbalanced port and theother end is connected to ground. Transmission line 14′ is grounded atboth ends. In this way, the coupler structure 10 is interconnected withcoupler structure 10′ such that in-phase port 2 and quadrature port 3are isolated from ground potential without decoupling components. Bothcoupler structures (10, 10′) as shown in FIG. 6 are implemented asvertical interdigital broadside couplers. However, those of ordinaryskill in the art will understand that the balun 100 may also beimplemented using edge coupled coupler structures.

Referring to FIG. 7, a chart illustrating the performance (insertionloss and return loss) of the balun 100 depicted in FIG. 6 is disclosed.From the discussion of the vertical interdigital coupler disclosedearlier (See, for example, FIG. 4), it becomes apparent that the finiteeven-mode impedance (Z_(e)) is largely dependent on device dimensions h,B. In the examples shown in FIG. 7, the finite even-mode impedance(Z_(e)) is kept relatively constant—the device profile remains the samefor each measurement. The finite odd-mode impedance (Z_(o)), on theother hand, is mostly dependant on dimensions d, w, s, and t. Theexample insertion loss and return loss curves shown in FIG. 7 are afunction of the finite odd-mode impedance (Z_(o)).

As shown in the chart, the bandwidth of interest is between point A (950MHz) and point B (2100 MHz). The insertion loss curves 700 within thespecified bandwidth varies from −4 dB at 950 MHZ to approximately 0 dBat 2100 MHz. In this example, an acceptable return loss must be −12 dBor better. Return loss curve 702 corresponds to a finite odd-modeimpedance (Z_(o)) of 34Ω. In the bandwidth region at or near 950 MHz,the return loss is approximately −11 dB, and therefore, the design isunacceptable. Return loss curve 704 and 706 correspond to a finiteodd-mode impedance (Z_(o)) of 30Ω and 32Ω respectively, and represent animprovement over curve 702. However, both show marginal performance inthe 950 MHz region.

Referring to FIG. 8, a chart illustrating the insertion loss of thebalun depicted in FIG. 6 as a function of frequency and even-modeimpedance is disclosed. The odd-mode impedance is fixed at 37.5 Ohms. Inthis embodiment, the three-dimensional graph 800 shows quite clearlythat the bandwidth is narrow at lower values of the even-mode impedance.The bandwidth reaches a maximum point at about 225 Ohms. Those ofordinary skill in the art will again understand that the profile heightof device 100 (FIG. 6) is proportional to the finite even-modeimpedance. At this point, the bandwidth extends between point 802 andpoint 804. As even-mode impedance increases, the bandwidth begins toexperience a decline.

Referring to FIG. 9, a chart illustrating the insertion loss of thebalun depicted in FIG. 6 as a function of frequency and odd-modeimpedance is disclosed. In FIG. 9, the even-mode impedance is fixed at225 Ohms. The bandwidth of the balun shown in FIG. 6 is limited forsmaller values of the odd-mode impedance. However, as shown by points(902, 904), the bandwidth is at a maximum at about 37.5 Ohms. Once theodd-mode impedance increases beyond approximately 42 Ohms, the bandwidthagain begins to decrease.

As embodied herein and depicted in FIG. 10, a balun in accordance withyet another embodiment of the present invention is disclosed. A balun110 in accordance with another embodiment of the present invention isdisclosed. This structure is similar to the device depicted in FIG. 6 inthat the balanced port is isolated from ground potential. Couplerstructure 10 includes coupled transmission line layers 12, 14, and 15.Transmission line layer 14 is disposed between transmission line layer12 and transmission line layer 15. Coupler structure 10′ is very similarto coupler structure 10. It includes coupled transmission line layers12′, 14′ and 15′. Transmission line layer 14′ is sandwiched between thetransmission line layer 12′ and transmission line layer 15′. Balun 110,of course, includes an unbalanced port 1 and a balanced port includingin-phase port 2 and quadrature phase port 3. The terms in-phase andquadrature, as used herein, merely refer to the fact that port 2 andport 3 accommodate signals that are of substantially equal in amplitudeand 180° out of phase with each other.

Unlike the previous embodiment, transmission line layer 12 is connectedto transmission line layer 15 and transmission line layer 12′ is alsoconnected to the transmission line layer 15′. The in-phase port (2) isconnected to transmission line layer 14 and quadrature port (3) isconnected to transmission line layer 14′. Further, the internal ends oftransmission line layer 14 and transmission line layer 14′ are connectedto each other. Transmission line layer 12 and transmission line layer 15have an outer end connected to ground potential and an internal endconnected to the unbalanced port. Transmission line layer 12′ andtransmission lines layer 15′ are connected to ground potential at bothends.

Referring to FIG. 11 and FIG. 12, the insertion loss performance of thebalun depicted in FIG. 10 is illustrated. FIG. 111 shows the insertionloss of the balun 110 as a function of frequency and even-modeimpedance. The odd-mode impedance is fixed at 37.5 Ohms. The balun isconfigured as a 75:75 Ohm balun. The performance of balun 110 is verysimilar to balun 100 (FIG. 6). At lower values of even-mode impedance,characterized by region 1102, the bandwidth provided by balun 110 isquite narrow. The available bandwidth of device 110 increases aseven-mode impedance increases, until the bandwidth reaches a maximum atabout 225 Ohms. Of course, the maximum bandwidth extends between points1104 and 1106. Those of ordinary skill in the art will understand thatthe device best operates, from an even mode standpoint, at about 225Ohms. Accordingly, any attempt to lower the device profile, such thateven-mode impedance is driven below the 225-250 Ohm region, will resultin severe bandwidth degradation.

FIG. 12 is a chart illustrating the insertion loss of the balun depictedin FIG. 10 as a function of frequency and odd-mode impedance. Theeven-mode impedance is fixed at 225 Ohms. The insertion loss experiencedby balun 110 at either end of the spectrum (50 MHz, 3950 MHz) tails offfor smaller values of the odd-mode impedance. However, the bandwidthreaches a maximum between points 1202 and 1204. The maximum correspondsto an odd-mode impedance of about 37.5 Ohms. Once the odd-mode impedancestarts to increase beyond the maximum, the insertion loss as a functionof frequency begins to decrease.

As embodied herein and depicted in FIG. 13, a balun 120 in accordancewith another embodiment of the present invention is disclosed. Couplerstructure 10 includes coupled transmission line layers 12, 14, and 15.Transmission line layer 14 is disposed between transmission line layer12 and transmission line layer 15. Coupler structure 10′ is very similarto coupler structure 10. It includes coupled transmission line layers12′, 14′ and 15′. Transmission line layer 14′ is disposed between thetransmission line layer 12′ and transmission line layer 15′. Balun 120includes an unbalanced port 1 and a balanced port including in-phaseport 2 and quadrature phase port 3. The terms in-phase and quadrature,as used herein, merely refer to the fact that port 2 and port 3accommodate signals that are of substantially equal in amplitude and180° out of phase with each other.

In this embodiment, in-phase port 2 is only connected to transmissionline layer 12. In similar fashion, quadrature port 3 is only connectedto transmission line layer 12′. The internal end of coupler 10transmission line 12 is connected to the internal end portion oftransmission layer 15′ and the internal end of transmission layer 15 isconnected to the internal end of transmission line layer 12′. One end oftransmission line 14 is connected to the unbalanced port and the otherend is connected to ground. Transmission line 14′ is grounded at bothends. As shown in FIG. 13, coupler structure 10 is interconnected withcoupler structure 10′ such that in-phase port 2 is DC isolated fromquadrature port 3 without any decoupling components, such as capacitorsor other such components typically employed.

Both coupler structures (10, 10′) as shown in FIG. 13 are implemented asvertical interdigital broadside couplers. However, those of ordinaryskill in the art will understand that the balun 100 may also beimplemented using edge coupled coupler structures.

Referring to FIG. 14, a chart 1400 illustrating the performance of balun120 (FIG. 13) is disclosed. Again, the bandwidth of interest is betweenpoint A (950 MHz) and point B (2100 MHz). Chart 1400 shows two deviceexamples, each having different finite odd-mode impedance (Z_(o))values. The first device is represented by insertion loss curve 1402 andreturn loss curve 1404. The second device is represented by insertionloss curve 1410 and return loss curve 1412. As before, an acceptablereturn loss must be −12 dB or better. The insertion loss 1402 of thefirst device is uneven over the specified bandwidth. In the 950 MHzregion the insertion loss exceeds −3 dB and is, therefore, unacceptable.The insertion loss 1402 is similarly degraded in the region approaching2150 MHz. The return loss curve 1404 is also problematic in the 950 MHzregion. However, by adjusting the finite odd-mode impedance (Z_(o)) inthe second example, the insertion loss curve 1410 and the return losscurve show marked improvement over the first example. The insertion loss1410 is substantially flat over the entire bandwidth. The return loss1412 is greater than −12 dB for the entire bandwidth.

Referring to FIG. 15, a chart illustrating the insertion loss of balun120 (FIG. 13) as a function of frequency and even-mode impedance isshown. Unlike previous embodiments, the insertion loss is at a minimumand the bandwidth at a maximum for smaller values of the even-modeimpedance. In fact, the even-mode impedance at the balun “sweet-spot” isapproximately one-half that of the previous balun embodiments (100,110). Accordingly, balun 120 represents a significant improvement from adevice profile reduction standpoint.

FIG. 16 is a chart illustrating the insertion loss balun 120 as afunction of frequency and odd-mode impedance. The maximum bandwidthextends between points 1602 and 1604. The odd-mode impedance at thispoint is approximately 30 Ohms.

In reference to the insertion loss charts shown in FIGS. 8, 9, 12, 12,15, and 16, it becomes apparent that the novel three (3) coupledtransmission line balun structures of the present invention represent animprovement over the related art. For example, in a two (2) coupledtransmission line Merrill balun, the even mode requirement is reducedfrom initially infinite to Z_(e)=3√{square root over (Z_(bal)Z_(se))}and further reduced to Z_(e)=√{square root over (2Z_(bal)Z_(se))}, or afactor of √{square root over (9)}/√{square root over (2)}≈2. Thetrade-off is an increase in the odd-mode impedance. However, the penaltyis relatively small. The increased odd mode impedance is at a rate ofonly √{square root over (2)}.

Referring to FIGS. 17A-17E, charts illustrating the transformation ratiotradeoffs of the present invention relative to a Merrill balun aredisclosed. In the Examples provided below, each chart is given analpha-numeric designation. The letter designation corresponds to thebalun type. For each letter designation, one (1) corresponds to a chartshowing the insertion loss as a function of the balanced impedance(Z_(bal)) and frequency for a relatively high even-mode impedance(Z_(e)). The high value of the even-mode impedance (Z_(e)) approximatesinfinity. The reader will also recognize that the odd-mode impedance(Z_(o)) is constant in all of the examples. Of course, in all of theexamples provided, the even-mode impedance (Z_(e)) and the odd-modeimpedance (Z_(o)) are a function of the balanced impedance (Z_(bal)) andthe single ended impedance (Z_(se)).

FIG. 17A-1 illustrates the insertion loss of a Merrill balun (FIG.17A-3) with the even-mode impedance (Z_(e)) being set at 1000. Theinsertion loss of the Merrill balun is acceptable at Z_(bal)=75 Ohms. InFIG. 17A-2, the even-mode impedance (Z_(e)) is reduced to three-timesthe geometric mean of the balanced impedance (Z_(bal)) and the singleended impedance (Z_(se)), i.e., Z_(e)=3√{square root over(Z_(bal)Z_(se))}. As shown, the reduction impairs the performance of thedevice at 75 Ohms. As the balanced impedance increases, the insertionloss as a function of frequency decreases.

FIG. 17B-1 illustrates the insertion loss of a balun 100 (See FIG. 6,FIG. 17B-3) with the even-mode impedance (Z_(e)) being set at 1000. InFIG. 17B-2, the even-mode impedance (Z_(e)) is again reduced. Thereduction of the even-mode impedance results in an improved wide-bandperformance of the device at 75 Ohms. Thus, the performance of thisembodiment is very strong at a 1:1 transformation ratio. However, as thebalanced impedance increases, the insertion loss as a function offrequency deteriorates rapidly.

FIG. 17C-1 illustrates the insertion loss of a balun 110 (See FIG. 10,FIG. 17C-3) with the even-mode impedance (Z_(e)) again being set at1000. In FIG. 17C-2, the even-mode impedance (Z_(e)) is again reduced.The performance of balun 110 shows remarkable flexibility. The insertionloss remains relatively flat as the balanced impedance (Z_(bal))increases.

FIG. 17D-1 illustrates the insertion loss of a balun 120 (See FIG. 13,FIG. 17D-3) with the even-mode impedance (Z_(e)) again being set at1000. However, in this example, the odd-mode impedance is three (3)times smaller that the previous examples. The performance of the deviceat these even-mode impedance and odd-mode impedance values is degradedwith respect to the previous examples. At 75 Ohms, the bandwidth isrelatively constricted. In FIG. 17D-2, the odd-mode impedance is furtherreduced and the even-mode impedance is reduced by more than half of thevalue of the previous examples (i.e., FIGS. 17A-2, 17B-2, 17C-2).According to FIG. 17D-2, balun 120 exhibits relatively good performancein the 75 Ohm region. The significance of this result becomes clear bycomparing FIG. 17B-2 with FIG. 17D-2. Balun 120 (FIG. 13, 17D-2) may beemployed to obtain comparable results while reducing the device profileheight in half relative to balun 100 (FIG. 6). FIGS. 17E-1 and 17E-2 areadditional illustrations of the performance of balun 120 (See FIG. 13,FIG. 17D-3). Comparing FIG. 17D-2 with FIG. 17E-2, it becomes apparentthat device 120 (FIG. 13) represents a significant reduction in devicesize relative to the other embodiments, while at the same time,providing acceptable performance in the 75 Ohm region.

As embodied herein and depicted in FIG. 18, a power divider 200 inaccordance with another embodiment of the present invention isdisclosed. In this embodiment, both coupler structure 10 and couplerstructure 10′ are identical to the coupler structures shown FIG. 8. Thepower divider 200 is implemented by connecting a resistor 20 betweencoupler 10′ and coupler structure 10′.

Power divider structure 200 is formed by connecting resistor element 20between transmission line layer 12 and transmission line layer 12′.Transmission line 12 is also internally connected to an end portion oftransmission layer 15′ and the internal end of transmission layer 15 issimilarly connected to the internal end of transmission line layer 12′.One end of transmission line 14 is connected to the unbalanced port andthe other end is connected to ground. Transmission line 14′ is groundedat both ends.

Power divider 200 includes unbalanced port 1, balanced port A andbalanced port B. Balanced port A includes in-phase port 2 connected totransmission line layer 12 and quadrature phase port 3 connected totransmission line layer 12′. Balanced port B includes in-phase port 4connected to transmission line layer 15 and quadrature phase port 5connected to transmission line layer 15′. Again, the terms in-phase andquadrature, as used herein, merely refer to the fact that port 2 andport 3 accommodate signals that are of substantially equal in amplitudeand 180° out of phase with each other. In any event, the signal directedinto the unbalanced port 1 is divided between balanced port A andbalanced B. In one embodiment, the signal provided to the unbalancedport 1 is split equally between the two balanced ports (A, B). However,those of ordinary skill in the art will understand that the signal maybe split unequally in accordance with any desired ratio.

As embodied herein and depicted in FIG. 19, a combiner 300 in accordancewith the present invention is disclosed. The embodiment depicted in FIG.11 is exactly the same device shown in FIG. 10. The only differencebetween the two devices is the manner in which they are being used. Inthe device shown in FIG. 10, an input signal is directed into unbalancedport 1. The signal is split two ways, and a first balanced signalappears at the output of balanced port A and a second balanced signalappears at the output of balanced port B. In FIG. 11, the outputs ofdifferential amplifier 50 are connected to balanced port A (2, 3) andthe outputs of differential amplifier B are connected to balanced port B(4, 5). The signals provided by the differential amplifiers (50, 52) arecombined and directed to the output at unbalanced port 1.

The combiner of the present invention is advantageous in that if one ofthe differential amplifiers experiences a fault condition and does notprovide a differential input signal, combiner 300 will continue toprovide an output signal, albeit at approximately half the magnitude.

Referring to FIG. 20, a perspective view of the device depicted ineither FIG. 6, 10, 13, 18 or 19 is disclosed. Each of these devices maybe implemented by interconnecting vertical interdigital coupler 10 andvertical interdigital coupler 10′ within a single compact housing.Coupler 10 occupies the upper-half of the device (100, 200, 300) andcoupler 10′ is disposed in the bottom portion of device (100, 200, 300).Coupler 10 and coupler 10′ share ground plate 18′. Thus, coupler 10 isdisposed between ground plate 18 and interior ground plate 18′ Coupler10′ is disposed between plate 18′ and lower ground plate 18″. Note thatthe device includes interior vias 30 configured to accommodate interiorsignal transmission paths. Those of ordinary skill in the art willunderstand that dielectric layers 16 (not shown) are disposed betweeneach transmission line 12, 14, 15 as well as 12′, 14′, 15′. Thedielectric layers 16 are not shown in FIG. 14 for clarity ofillustration.

Referring to FIG. 21, an exploded view of the device depicted in eitherFIG. 6, 10, 13, 18 or 19 is disclosed without any of theinterconnections. Coupler 10 and coupler 10′ are identicalthree-transmission layer devices, i.e., each vertical interdigitalcoupler 10 (10′) includes three coupled transmission lines 12, 14, 15(12′, 14′, 15′). Again, each transmission line is disposed on adielectric substrate 16 (not shown in this view).

In general, each coupler structure (10, 10′) of the present inventionmay be fabricated in the following manner. As an initial step, thegeometric configuration, i.e., the shape of the transmission line inplan view, the width of the conductors, the thickness of the conductors,and all the various spacing dimensions have been calculated. Eachtransmission line layer is provided as a conductive sheet bonded to adielectric sheet. Subsequently, the predetermined geometric pattern istransferred to the surface of the conductive sheet usingphotolithographic techniques. A photoresist material is disposed on theconductive sheet and the pattern is transferred to the resist materialby directing radiant energy through a mask. The mask, of course,includes the image of the pattern. Imaging optics disposed in thephotolithographic system ensure that the line widths transferred to thesurface of the photoresist are properly dimensioned within anappropriate tolerance range. Subsequently, the exposed photoresistmaterial and the underlying portion of the conductive sheet are removedby applying an etchant. The etching provides the transmission line layerwherein a transmission line is disposed on a dielectric substrate 16.

With respect to coupler structure 10, transmission line layers 12, 14,and 15 are placed in vertical alignment with each other using a suitableregistration method. For example, those of ordinary skill in the artwill understand that various keying structures and techniques may beemployed to ensure that vertical alignment is effected. After alignment,the transmission line layers 12, 14, 15 are bonded together to form alaminate structure. Again, those of ordinary skill in the art willunderstand that any suitable bonding technique may be employed dependingon the type of dielectric material used to implement dielectric layer16. For example, with certain polymer dielectric materials, the step ofbonding may be performed by applying heat and/or pressure to thesandwiched transmission line layers. After lamination is completed, thetransmission line layers are interconnected in accordance with schematicdiagrams shown in FIGS. 6, 8, 10, and 11.

Reference is made to U.S. patent application Ser. No. 11/419,091, filedon May 18, 2006, which is incorporated herein by reference as thoughfully set forth in its entirety, for a more detailed explanation of thevertical interdigital couplers used herein.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. The term “connected” is to beconstrued as partly or wholly contained within, attached to, or joinedtogether, even if there is something intervening.

The recitation of ranges of values herein are merely intended to serveas a shorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein.

All methods described herein can be performed in any suitable orderunless otherwise indicated herein or otherwise clearly contradicted bycontext. The use of any and all examples, or exemplary language (e.g.,“such as”) provided herein, is intended merely to better illuminateembodiments of the invention and does not impose a limitation on thescope of the invention unless otherwise claimed.

No language in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. There isno intention to limit the invention to the specific form or formsdisclosed, but on the contrary, the invention is to cover allmodifications, alternative constructions, and equivalents falling withinthe spirit and scope of the invention, as defined in the appendedclaims. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

1. A balun comprising: a first coupler structure including a first port of a balanced port pair and an unbalanced port; a second coupler structure including a second port of the balanced port pair, the second coupler structure being connected to the first coupler structure such that the second port of the balanced port pair is DC isolated from the first port of the balanced port pair without decoupling components.
 2. The balun of claim 1, wherein the first coupler structure includes a first transmission line layer coupled to a second transmission line layer, and a third transmission line layer coupled to the second transmission layer, the second transmission line layer being sandwiched between the first transmission line layer and the third transmission line layer, and wherein the second coupler structure including a fourth transmission line layer coupled to a fifth transmission line layer, and a sixth transmission line layer coupled to the fifth transmission layer, the fifth transmission line layer being sandwiched between the fourth transmission line layer and the sixth transmission line layer, and wherein the first transmission line layer is connected to the sixth transmission line layer and wherein the third transmission line layer is connected to the fourth transmission line layer.
 3. The balun of claim 2, wherein the first port of a balanced port pair is connected to the first transmission line layer and the second port of the balanced port pair is connected to the fourth transmission line layer.
 4. The balun of claim 2, wherein the second transmission line layer has a first end connected to ground potential and a second end connected to the unbalanced port.
 5. The balun of claim 2, wherein the fifth transmission line is grounded at both ends.
 6. The balun of claim 2, wherein each transmission line layer includes a conductive transmission line conforming to a predetermined geometric configuration, the conductive transmission line being disposed on a dielectric material.
 7. The balun of claim 6, wherein the predetermined geometric configuration includes a planar pattern (X-Y plane) selected from a group of planar patterns that includes linear planar patterns, non-linear planar patterns, spiral patterns, substantially rectangular patterns.
 8. The balun of claim 2, further comprising a resistive element connected between the first transmission line layer and the fourth transmission line layer to form a power combiner/divider structure.
 9. The balun of claim 8, wherein the first port of the balanced port pair is connected to the first transmission line layer and the second port of the balanced port pair is connected to the fourth transmission line layer and a first port of a second balanced port pair is connected to the third transmission line and a second port of the second balanced port pair is connected to the sixth transmission line.
 10. The balun of claim 1, wherein the first coupler structure and the second coupler structure are each characterized by a physical coupler form factor based on a cross-sectional area, a predetermined geometrical configuration, and a selected coupling constant.
 11. The balun of claim 10, wherein the cross-sectional area is proportional to: A _(N)=(s+w)[2h+(N−1)d+Ntm]; and wherein s is a horizontal spacing between adjacent transmission line conductors, w is a horizontal width of each transmission line conductor, h is a vertical distance from an outermost transmission line conductor to a ground plane, N is a total number of transmission line layers in each of the first coupler structure and the second coupler structure, d is a vertical distance between sandwiched transmission line conductors, t is a vertical thickness of each transmission line conductor, and m is a ratio in a horizontal direction of conducting material to dielectric material.
 12. The balun of claim 11, wherein the first coupler structure and the second coupler structure are characterized by a finite even-mode impedance (Z_(e)) and a finite odd-mode impedance (Z_(o)), the ratio (R=Z_(e)/Z_(o)) being greater than or equal to one (1).
 13. The balun of claim 12, wherein Z_(o) is a function of d, w s, and t.
 14. The balun of claim 12, wherein Z_(e) is a function of at least h.
 15. The balun of claim 11, wherein N is equal to three (3).
 16. The balun of claim 10, wherein the predetermined geometric configuration is substantially linear.
 17. The balun of claim 10, wherein the predetermined geometric configuration includes at least one substantially rectangular geometric pattern.
 18. The balun of claim 10, wherein the predetermined geometric configuration is a non-linear geometric configuration.
 19. The balun of claim 10, wherein the predetermined geometric configuration includes at least one meandered line segment.
 20. The balun of claim 10, wherein the predetermined geometric configuration includes a spiral configuration.
 21. The balun of claim 1, wherein the first coupler structure and the second coupler structure are characterized by a finite even-mode impedance (Z_(e)) and a finite odd-mode impedance (Z_(o)), the ratio (R=Z_(e)/Z_(o)) being greater than or equal to one (1).
 22. The balun of claim 1, wherein the first coupler structure is a vertical interdigital coupler structure and the second coupler structure is a vertical interdigital coupler structure.
 23. The balun of claim 22, wherein the vertical interdigital coupler structures comprise broadside couplers.
 24. The balun of claim 1, wherein the first coupler structure is a vertical interdigital coupler structure and the second coupler structure comprise edge-coupled structures.
 25. A balun comprising: a first coupler structure including a first port of a balanced port pair and an unbalanced port; and a second coupler structure including a second port of the balanced port pair, the second coupler structure being connected to the first coupler structure such that the first port of the balanced port pair and the second port of the balanced port pair are isolated from ground potential without decoupling components.
 26. The balun of claim 25, wherein the first coupler structure includes a first transmission line layer coupled to a second transmission line layer, and a third transmission line layer coupled to the second transmission layer, the second transmission line layer being sandwiched between the first transmission line layer and the third transmission line layer, and wherein the second coupler structure including a fourth transmission line layer coupled to a fifth transmission line layer, and a sixth transmission line layer coupled to the fifth transmission layer, the fifth transmission line layer being sandwiched between the fourth transmission line layer and the sixth transmission line layer, and wherein the first transmission line layer is connected to the third transmission line layer and the fourth transmission line layer and wherein the fourth transmission line layer is also connected to the sixth transmission line layer.
 27. The balun of claim 26, wherein the first port of a balanced port pair is connected to the first transmission line layer and the second port of the balanced port pair is connected to the fourth transmission line layer.
 28. The balun of claim 26, wherein the second transmission line layer has a first end connected to ground potential and a second end connected to the unbalanced port.
 29. The balun of claim 26, wherein the fifth transmission line is grounded at both ends.
 30. The balun of claim 25, wherein the first coupler structure includes a first transmission line layer coupled to a second transmission line layer, and a third transmission line layer coupled to the second transmission layer, the second transmission line layer being disposed between the first transmission line layer and the third transmission line layer, and wherein the second coupler structure including a fourth transmission line layer coupled to a fifth transmission line layer, and a sixth transmission line layer coupled to the fifth transmission layer, the fifth transmission line layer being disposed between the fourth transmission line layer and the sixth transmission line layer, and wherein the first transmission line layer is connected to the third transmission line layer and wherein the fourth transmission line layer is also connected to the sixth transmission line layer.
 31. The balun of claim 30, wherein the first port of a balanced port pair is connected to the second transmission line layer and the second port of the balanced port pair is connected to the fifth transmission line layer, and wherein the second transmission line layer is connected to the fifth transmission line layer.
 32. The balun of claim 30, wherein the first transmission line layer and the third transmission line layer have a first end connected to ground potential and a second end connected to the unbalanced port.
 33. The balun of claim 30, wherein the fourth transmission line layer and the sixth transmission lines layer are grounded at both ends.
 34. A device configured to operate within a predetermined band of frequencies, the device comprising: a first coupler structure including a first portion of a first balanced port, a first port of a second balanced port and an unbalanced port; a resistive element connected to the first coupler structure; and a second coupler structure including a second portion of the first balanced port and a second portion of the second balanced port, the second coupler structure being connected to the first coupler structure by way of the resistive element such that the first and second portions of the first balanced port pair and the first and second portions of the second balanced port pair are isolated from each other substantially within the predetermined band without decoupling components.
 35. The device of claim 34, wherein the first coupler structure includes a first transmission line layer coupled to a second transmission line layer and a third transmission line layer coupled to the second transmission layer, the second transmission line layer being disposed between the first transmission line layer and the third transmission line layer, the second coupler structure including a fourth transmission line layer coupled to a fifth transmission line layer and a sixth transmission line layer coupled to the fifth transmission layer, the fifth transmission line layer being disposed between the fourth transmission line layer and the sixth transmission line layer, the first transmission line layer being connected to the sixth transmission line layer and the third transmission line layer being connected to the fourth transmission line layer.
 36. The device of claim 35, wherein the first portion of the first balanced port pair is connected to the first transmission line layer and the second portion of the first balanced port pair is connected to the fourth transmission line layer.
 37. The device of claim 35, wherein the first portion of the second balanced port pair is connected to the third transmission line layer and the second portion of the second balanced port pair is connected to the sixth transmission line layer.
 38. The device of claim 35, wherein the resistive element is disposed between the first transmission line layer and the fourth transmission line layer.
 39. The device of claim 35, wherein the second transmission line layer has a first end connected to ground potential and a second end connected to the unbalanced port.
 40. The device of claim 35, wherein the fifth transmission line is grounded at both ends.
 41. The device of claim 34, wherein the sum of the signal power of the first balanced port and the signal power of the second balanced port is substantially equal to the signal power of the unbalanced port.
 42. The device of claim 34, wherein the unbalanced port is configured as an input port and the first balanced port and the second balanced port are configured as output ports.
 43. The device of claim 34, wherein the unbalanced port is configured as an output port and the first balanced port and the second balanced port are configured as input ports.
 44. A balun comprising: a first coupler structure including a first port of a balanced port pair and an unbalanced port, the first coupler structure includes a first transmission line layer coupled to a second transmission line layer and a third transmission line layer coupled to the second transmission layer, the second transmission line layer being disposed between the first transmission line layer and the third transmission line layer; a second coupler structure including a second port of the balanced port pair, the second coupler structure also including a fourth transmission line layer coupled to a fifth transmission line layer and a sixth transmission line layer coupled to the fifth transmission layer, the fifth transmission line layer being disposed between the fourth transmission line layer and the sixth transmission line layer, the first transmission line layer being connected to the sixth transmission line layer and the third transmission line layer being connected to the fourth transmission line layer such that the first port of the balanced port pair is DC isolated from the second port of the balanced port pair.
 45. The balun of claim 44, wherein the first port of a balanced port pair is connected to the first transmission line layer and the second port of the balanced port pair is connected to the fourth transmission line layer.
 46. The balun of claim 44, wherein the second transmission line layer has a first end connected to ground potential and a second end connected to the unbalanced port.
 47. The balun of claim 44, wherein the fifth transmission line is grounded at both ends.
 48. The balun of claim 44, wherein the first port of the balanced port pair is DC isolated from the second port of the balanced port pair without decoupling components.
 49. The balun of claim 44, wherein each transmission line layer includes a conductive transmission line disposed on a dielectric material.
 50. The balun of claim 44, wherein the first coupler structure and the second coupler structure are arranged as vertically interdigital coupler structures. 